That is strange. I swear it didn't show up in game - I know because I was particularly enthusiastic about the voice line I selected for it but it never plays. Didn't get any other achievement at the same time with higher priority. I'm pretty sure it didn't show up on the scoreboard either as my last achievement. It's odd that it shows up in the saved session though. Does the saved session parse the data independently to determine achievements after each kill or is the achievement data sent too when the session is uploaded?